Ah, yes grimlock, king of the dinobots, the only transformer to go from autobot, to decepticon, to autobot, to terrorcon, to autobot. And he still hates humans, except spike and his kid.
But the overall coolness factor for a transformer has to go to starscream, he was (at least according to pre beast wars continuity) a former autobot scientist (and pal of Jetfire, btw) who joined the decepticons originally to gain control and settle the autobot/decepticon war, unfortunatly greed got the better of him, but he still has his moments. Also he is the only undead transformer (being a ghost), and how cool is that). He is also the only transformer to show up in all series thus far (including beast wars).
DECEPTICONS ATTACK!!! RETREAT!!!! -Starscream.
Later,